Lithographic apparatus, device manufacturing method, and device manufactured thereby

ABSTRACT

A lithographic apparatus is disclosed. The apparatus includes a radiation system that provides a beam of radiation, and a support structure that supports a patterning structure. The patterning structure is configured to pattern the beam of radiation according to a desired pattern. The apparatus also includes a substrate support that supports a substrate, and a projection system that projects the patterned beam onto a target portion of the substrate. The projection system includes an optical element that has a beam entry area and an optical element that has a beam exit area through each of which the patterned beam passes. The apparatus further includes a nucleated surface that is associated with the projection system on which a plurality of nucleation sites are provided. The surface is disposed away from at least one of the beam entry area and the beam exit area.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 10/748,751, filed Dec. 31, 2003 and currently pending, the entirecontent of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a lithographic projectionapparatus, and more specifically to a lithographic projection apparatuswith a fluid cleaning system.

2. Description of Related Art

The term “patterning device” or “patterning structure” as here employedshould be broadly interpreted as referring to a device or structure thatcan be used to endow an incoming radiation beam with a patternedcross-section, corresponding to a pattern that is to be created in atarget portion of the substrate. The term “light valve” can also be usedin this context. Generally, the said pattern will correspond to aparticular functional layer in a device being created in the targetportion, such as an integrated circuit or other device (see below).Examples of such patterning devices include:

-   -   A mask. The concept of a mask is well known in lithography, and        it includes mask types such as binary, alternating phase-shift,        and attenuated phase-shift, as well as various hybrid mask        types. Placement of such a mask in the radiation beam causes        selective transmission (in the case of a transmissive mask) or        reflection (in the case of a reflective mask) of the radiation        impinging on the mask, according to the pattern on the mask. In        the case of a mask, the support structure will generally be a        mask table, which ensures that the mask can be held at a desired        position in the incoming radiation beam, and that it can be        moved relative to the beam if so desired;    -   A programmable mirror array. One example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the said undiffracted light        can be filtered out of the reflected beam, leaving only the        diffracted light behind. In this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. An alternative embodiment of a        programmable mirror array employs a matrix arrangement of tiny        mirrors, each of which can be individually tilted about an axis        by applying a suitable localized electric field, or by employing        a piezoelectric actuation device. Once again, the mirrors are        matrix-addressable, such that addressed mirrors will reflect an        incoming radiation beam in a different direction to unaddressed        mirrors; in this manner, the reflected beam is patterned        according to the addressing pattern of the matrix-addressable        mirrors. The required matrix addressing can be performed using        suitable electronic means. In both of the situations described        hereabove, the patterning device can comprise one or more        programmable mirror arrays. More information on mirror arrays as        here referred to can be gleaned, for example, from United States        Patents U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and        PCT patent applications WO 98/38597 and WO 98/33096, which are        incorporated herein by reference. In the case of a programmable        mirror array, the said support structure may be embodied as a        frame or table, for example, which may be fixed or movable as        required; and    -   A programmable LCD array. An example of such a construction is        given in United States Patent U.S. Pat. No. 5,229,872, which is        incorporated herein by reference. As above, the support        structure in this case may be embodied as a frame or table, for        example, which may be fixed or movable as required.        For purposes of simplicity, the rest of this text may, at        certain locations, specifically direct itself to examples        involving a mask and mask table. However, the general principles        discussed in such instances should be seen in the broader        context of the patterning device as hereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion in one go; such an apparatus is commonly referred toas a wafer stepper or step and repeat apparatus. In an alternativeapparatus—commonly referred to as a step and scan apparatus—each targetportion is irradiated by progressively scanning the mask pattern underthe projection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti parallel to this direction; since, in general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be gleaned, for example, fromU.S. Pat. No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion implantation (doping), metallization, oxidation, chemomechanical polishing, etc., all intended to finish off an individuallayer. If several layers are required, then the whole procedure, or avariant thereof, will have to be repeated for each new layer.Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0 07 067250 4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”. However, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, both incorporated herein by reference.

It has been found that used G- and I-line and deep UV microlithographylenses suffer from degradation in terms of loss of overall transmissionand loss of wafer illumination uniformity.

In a purged system, i.e., a system that is purged with a purging gas,this degradation is mainly caused by the occurrence of contamination onthe surfaces of the first and last optical element in the projectionsystem, i.e., the first optical element encountered by the projectionbeam and the last optical element encountered by the projection beam inthe projection system. It will be understood, however, that in systemsthat are not purged, crystal growth is likely on other surfaces of theprojection system in addition to the surfaces of the first and the lastoptical elements. Such contamination comprises dendritic salt structureswhich grow on the lens surfaces. It has been found that lenses subjectto intense radiation over a period of time, typically a few years,become contaminated with salt structures. This problem is not limited tothe particular type of radiation used, but has been found to occur withradiation of 365 nm, 248 nm, 193 nm, 157 nm as well as extreme ultraviolet (EUV) lithography. It is mentioned that EUV lithography apparatusare typically not purged systems. The origin of the lens surfacecontamination appears to be refractory compounds, such as silane, beingpresent at very low concentrations, i.e., parts per million (ppm) toparts per billion (ppb) in the purge air, which is used as a medium inthe lithographic apparatus to stabilize conditions within the apparatus,and have even been found in purified nitrogen used for special purgingpurposes. Irradiation induced chemical surface reactions of silanes,sulphates or phosphates in combination with the presence of other gases,such as, for example, oxygen, water, and ammonia, are considered to bethe basis degradation mechanism. It is believed that nucleation as wellas growth of the contaminating crystals occur during exposure withradiation of the G-, I-, deep UV and EUV wavelengths. It is believedthat these wavelengths, at least, cause a particular photochemicalreaction to occur.

Conventionally, this problem has been addressed by mechanical orchemical cleaning with a non-scratching cloth wetted with specificchemicals. It has been found, however, that this conventional approachresults in a spreading or distribution of salt growth nuclei over theentire lens surface. Subsequent use of the lens in the projection systemresults in accelerated growth of the contamination over the entire“cleaned” lens surface. This effect dramatically reduces the opticalthroughput, the optical imaging quality and the time between subsequentcleaning. After a number of cleaning rounds, it has been found thatremoval of the surface contamination becomes more difficult. Theoccurrence of contamination may finally require a complete interchangeof the dirty projection system with a new system, which is veryexpensive.

The problem of removing contaminants from cooling air is addressed inU.S. Pat. No. 5,696,623, which discloses an air purge system including amethod for cleaning cooling gas in a semiconductor manufacturing device.The method includes exposing the cooling air to ultraviolet radiation.One problem with this particular prior art is that it is necessary tocool the air. It has been found that exposing air to ultraviolet priorto it being passed through the lens system does not eliminate saltcrystal growth.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to address the problemsencountered with conventional fluid cleaning systems.

This and other aspects are achieved according to embodiments of theinvention in a lithographic apparatus comprising: a radiation system forproviding a projection beam of radiation; a support structure forsupporting the patterning device, the patterning device serving topattern the projection beam according to a desired pattern; a substratetable for holding a substrate; and a projection system for projectingthe patterned beam onto a target portion of the substrate, theprojection system including an optical element having a beam entry areaand an optical element having a beam exit area through each of which thepatterned beam passes, characterized by a nucleated surface on which aplurality of nucleation sites are provided with one of which acontaminant present in or around said projection system associates, saidsurface being disposed away from at least one of said beam entry area orsaid beam exit area.

In an embodiment, a lithographic projection apparatus is provided. Theapparatus includes a radiation system that provides a beam of radiation,and a support structure that supports a patterning structure. Thepatterning structure is configured to pattern the beam of radiationaccording to a desired pattern. The apparatus also includes a substratesupport that supports a substrate, and a projection system that projectsthe patterned beam onto a target portion of the substrate. Theprojection system includes an optical element that has a beam entry areaand an optical element that has a beam exit area through each of whichthe patterned beam passes. The apparatus further includes a nucleatedsurface that is associated with the projection system on which aplurality of nucleation sites are provided. The surface is disposed awayfrom at least one of the beam entry area and the beam exit area.

This arrangement provides an advantage in that a contaminating saltgrowth is eliminated from an optical element in the projection system,while causing minimal impact to the performance of the apparatus. Afurther advantage is that cooling of the fluid is not necessary toachieve cleaning of it.

According to a further aspect of the present invention, the nucleatedsurface is made of the same material as at least one of the opticalelements.

This arrangement provides an advantage that the nucleated surface actsas a “dummy” surface, that is, contaminants that would otherwisecontaminate the optical element, due to the particular material of theoptical element, contaminate the “dummy” surface instead. By associationwith the nucleated surface, the contaminant is retained on the dummysurface, thus preventing any further contamination by the samecontaminant.

In an embodiment, a lithographic projection apparatus is provided. Theapparatus includes a first radiation system that provides a beam ofradiation and a support structure that supports a patterning structure.The patterning structure is configured to pattern the projection beamaccording to a desired pattern. The apparatus also includes a substratesupport that supports a substrate, and a projection system that projectsthe patterned beam onto a target portion of the substrate. Theprojection system includes an optical element that has a beam entry areaand an optical element that has a beam exit area through each of whichthe patterned beam passes. The apparatus further includes a fluidcleaning system that cleans a fluid to be introduced into a region inwhich the optical element is disposed. The fluid cleaning systemincludes a fluid inlet that receives fluid to be cleaned and a fluidoutlet that supplies cleaned fluid to the region of the apparatus, acleaning zone that cleans the received fluid, the cleaning zone beingdisposed between said inlet and said outlet, and a second radiationsystem that provides radiation to the cleaning zone to causedissociation of a contaminant present in the fluid in the cleaning zone.The apparatus also includes a nucleated surface provided with aplurality of nucleation sites. The nucleated surface is disposed in thecleaning zone.

According to a further aspect there is provided a fluid cleaning systemfor use in an apparatus, said system comprising: a fluid inlet forreceiving gas to be cleaned and a fluid outlet and supply system forsupplying cleaned fluid to an apparatus; a cleaning zone disposedbetween said inlet and said outlet; and a radiation source arranged, inuse, to be incident on said cleaning zone; characterized in that saidradiation source causes dissociation of a contaminant present in saidfluid in said cleaning zone; and in that said fluid cleaning systemfurther comprises: a nucleated surface disposed in said cleaning zone,on which a plurality of nucleation sites are provided with one of whichsaid dissociated contaminant associates.

In an embodiment, a fluid cleaning system for use in an apparatus isprovided. The system includes a fluid inlet that receives fluid to becleaned and a fluid outlet that supplies cleaned fluid to an apparatus,a cleaning zone disposed between the inlet and the outlet, a radiationsource arranged to be incident on the cleaning zone to causedissociation of a contaminant present in the fluid in the cleaning zone,and a nucleated surface disposed in the cleaning zone, on which aplurality of nucleation sites are provided.

In an embodiment, a method of cleaning a fluid for use in an apparatusis provided, The method includes receiving a fluid to be cleaned at aninlet and supplying a cleaned fluid to an apparatus at an outlet,cleaning the fluid in a cleaning zone disposed between the inlet and theoutlet, using a radiation source to cause dissociation of a contaminantpresent in the fluid in the cleaning zone, and providing a nucleatedsurface in the cleaning zone, on which a plurality of nucleation sitesare provided.

This arrangement provides the advantage in that the fluid is cleaned toultra high standards because the contaminant is retained in the cleaningzone. In particular, salt crystal growth which, after time, damagesexpensive apparatus components is eliminated. Thus, the life time of theapparatus is increased. The costs of the fluid cleaning system arelimited because there is no precision optics required and no precisetuning of the fluid cleaning system or the apparatus is required.

According to a further aspect of the invention there is provided adevice manufacturing method comprising the steps of: providing asubstrate that is at least partially covered by a layer ofradiation-sensitive material; providing a projection beam of radiationusing a radiation system; using a patterning device to endow theprojection beam with a pattern in its cross-section; and projecting thepatterned beam of radiation using an optical element having a beam entryarea and an optical element having a beam exit area through each ofwhich said patterned beam passes, onto a target portion of the layer ofradiation-sensitive material, characterized by providing a nucleatedsurface on which a plurality of nucleation sites are provided with oneof which a dissociated contaminant present in or around said projectionsystem associates, and disposing said surface away from at least one ofsaid beam entry area or said beam exit area.

In an embodiment, a device manufacturing method is provided. The methodincludes projecting a beam of radiation, patterning the beam ofradiation, projecting the patterned beam of radiation using an opticalelement having a beam entry area and an optical element having a beamexit area through each of which the patterned beam passes, onto a targetportion of the layer of radiation-sensitive material, and capturingcontaminants with a plurality of nucleation sites spaced from at leastone of the beam entry area and the beam exit area. A dissociatedcontaminant present in or around a projection system associates with atleast one of the plurality of nucleation sites.

In an embodiment, a contamination detector for detecting contaminants ina fluid is provided. The detector includes a fluid path along which thefluid flows, a detection zone disposed in said fluid path, and aradiation source arranged to be incident on the detection zone. Theradiation source causes dissociation of a contaminant present in thefluid in the detection zone. The detector also includes a nucleatedsurface disposed in the detection zone, on which a plurality ofnucleation sites are provided; and an optical measuring device fordetermining an optical characteristic of the nucleated surface fromwhich concentration of a contaminant in said fluid is determined.

Although specific reference may be made in this text to the use of theapparatus according to embodiments of the invention in the manufactureof ICs, it should be explicitly understood that such an apparatus hasmany other possible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid crystal display panels,thin film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultraviolet(UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm)and extreme ultra-violet (EUV) radiation (e.g. having a wavelength inthe range 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

FIG. 2 depicts details of a lithographic apparatus including aprojection system protector according to an embodiment of the presentinvention;

FIG. 3 depicts details of a lithographic apparatus including aprojection system protector according to a further embodiment of thepresent invention;

FIG. 4 depicts details of a lithographic apparatus including aprojection system protector and attachments according to a furtherembodiment of the present invention;

FIG. 5 depicts details of a lithographic apparatus including a fluidcleaning system according to an embodiment of the present invention;

FIG. 6 depicts a fluid cleaning system according to an embodiment of thepresent invention;

FIG. 7 depicts a fluid cleaning system according to a further embodimentof the present invention; and

FIG. 8 depicts details of a lithographic apparatus including a fluidcleaning system according to a further embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 schematically depicts a lithographic projection apparatus 1according to a particular embodiment of the invention. The apparatuscomprises: a radiation system Ex, IL, for supplying a projection beam PBof radiation (e.g. 365, 248, 193, 157 nm radiation). In this particularcase, the radiation system also comprises a radiation source LA; a firstobject table (mask table) MT provided with a mask holder for holding amask MA (e.g. a reticle), and connected to a first positioning devicefor accurately positioning the mask with respect to item PL; a secondobject table (substrate table) WT provided with a substrate holder forholding a substrate W (e.g. a resist coated silicon wafer), andconnected to a second positioning device for accurately positioning thesubstrate with respect to item PL; and a projection system (“lens”) PL(e.g. an optical lens system) for imaging an irradiated portion of themask MA onto a target portion C (e.g. comprising one or more dies) ofthe substrate W. The term “object table” as used herein may also beconsidered or termed as an object support. It should be understood thatthe term object support or object table broadly refers to a structurethat supports, holds, or carries an object or substrate.

As here depicted, the apparatus is of a transmissive type (i.e. has atransmissive mask). However, in general, it may also be of a reflectivetype, for example, with a reflective mask. Alternatively, the apparatusmay employ another kind of patterning device, such as a programmablemirror array of a type referred to above.

The source LA (e.g. a mercury lamp, a Krypton Fluoride excimer laser ora plasma source) produces a beam of radiation. This beam is fed into anillumination system (illuminator) IL, either directly or after havingtraversed conditioning means, such as a beam expander Ex, for example.The illuminator IL may comprise an adjusting device AM for setting theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in the beam. Inaddition, it will generally comprise various other components, such asan integrator IN and a condenser CO. In this way, the beam PB impingingon the mask MA has a desired uniformity and intensity distribution inits cross section.

It should be noted with regard to FIG. 1 that the source LA may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, with theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors). This latter scenario is oftenthe case when the source LA is an excimer laser. The current inventionand claims encompass both of these scenarios.

The beam PB subsequently intercepts the mask MA, which is held on a masktable MT. Having traversed the mask MA, the beam PB passes through thelens PL, which focuses the beam PB onto a target portion C of thesubstrate W. With the aid of the second positioning device (and aninterferometric measuring device IF), the substrate table WT can bemoved accurately, e.g., so as to position different target portions C inthe path of the beam PB. Similarly, the first positioning device can beused to accurately position the mask MA with respect to the path of thebeam PB, e.g., after mechanical retrieval of the mask MA from a masklibrary, or during a scan. In general, movement of the object tables MT,WT will be realized with the aid of a long-stroke module (coarsepositioning) and a short-stroke module (fine positioning), which are notexplicitly depicted in FIG. 1. However, in the case of a wafer stepper(as opposed to a step-and-scan apparatus) the mask table MT may just beconnected to a short stroke actuator, or may be fixed. Mask MA andsubstrate W may be aligned using mask alignment marks and substratealignment marks.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected in one go (i.e. a single “flash”) ontoa target portion C. The substrate table WT is then shifted in the xand/or y directions so that a different target portion C can beirradiated by the beam PB; and

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so called “scandirection”, e.g. the y direction) with a speed ν, so that the projectionbeam PB is caused to scan over a mask image. Concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mν, in which M is the magnification of the lens PL (typically,M=¼ or ⅕). In this manner, a relatively large target portion C can beexposed, without having to compromise on resolution.

As mentioned, the projection system (“lens”) PL, is, for example, anoptical lens system for imaging an irradiated portion of the mask MAonto a target portion C. The optical lens system typically includesaround thirty lens elements arranged so that the projection beam passesthrough each lens element. Each lens element has two surfaces throughwhich the projection beam passes. It has been found that the salt growthis most marked on the external lens surfaces, that is the first surfacethe projection beam passes through on entry into the projection systemand the last surface the projection passes through on exiting theprojection system. In particular, the projection system includes anoptical element having a beam entry area and an optical element having abeam exit area through which said projection beam passes on entry intothe projection system and exit out of the projection system,respectively.

The present invention has application to the cleaning of fluids,including gases as well as liquids. In particular, but not exclusively,the invention has application to the cleaning of gases in opticallithography and to the cleaning of liquids in immersion lithography.

Before describing further details of embodiments of the invention, thephotochemical reaction is described. It is known from, for example, selfassembled monolayering, that functional groups adhere preferably at agiven surface, for example, sulphur containing molecules on gold,alcohol and amide groups on platinum, fatty acid groups on silver andsilicon dioxide, 1-alkenes on silicon, and alkyl(di)phosphoric acids onmica. The typical contaminants include compounds comprising silicon,calcium, sulphur, phosphorus, aluminium and other metals, furtherincluding sulphur dioxide, ammonium sulphate, phosphoric acid, silanesor compounds having a hydrocarbon or fluorocarbon tail including allorganic metal complexes. In most cases, only a small part of thecompounds, such as an atom or a functional group, remains as thenucleation site for the crystal. The residual (non-nucleation siteforming) part, which is typically organic or hydrocarbon is oxidizedaway in the lithographic apparatus. The salt crystals are known, forexample, as “whiskers”, nano-tubes or nano-wires, dentrites orrefractory compounds. Nucleation as well as growth of crystals derivedfrom the contaminant occurs during exposure with intense radiation, suchas that found in lithographic apparatus and apparatus using electronradiation.

Irradiation induced chemical reactions of chemicals, such as silanes,sulphates or phosphates in combination with the presence of other gasessuch as oxygen, water, ammonia, is the basic contamination salt growthmechanism. Firstly, exposure to the radiation causes the silanes,sulphates or phosphates to dissociate. Secondly, after dissociation, thechemicals reform as other compounds, specifically salt compounds. It hasbeen found that the wavelength in the lithography apparatus producesparticular chemical reactions in the environment, because salt compoundswould not typically occur in the environment found in the lithographicapparatus, that is an environment of purified, clean, dry air, withoutthe presence of the intense radiation. It has been found that a decreasein the wavelength of the projection beam from for example, 365 nm to 193nm, EUV and electron irradiation, provides additional energy, which inturn provides more dissociation and, hence more of the damaging saltcompounds. Typical salt crystal compounds formed include, for example,magnesium sulphate, magnesium phosphate and ammonium sulphate. Thesesalts, in conventional apparatus deposit onto the lens elements that thelight passes through. When the light travels through a lens followingsalt contamination of that lens, it causes a diffraction of the lightpassing through the lens. This diffraction of light, also referred to asstray light or flare, causes problems within the lithography process.Salt crystal compounds are formed by rearranging atoms or molecules froma fluid or solution state into ordered solid state. It has been foundthat in conventional systems, the nucleation mechanism arises as aresult of lens imperfections plus impurities. It has also been foundthat nucleation is needed for crystal growth. However, nucleation takesa lot longer than crystal growth. Thus, once a surface has beennucleated, crystal growth progresses rapidly.

The two major contaminants are sulphur dioxide, which is found in theambient air, and phosphates from volatile organo-phosphates, which arepresent in plastics in the lithographic apparatus, for example, asplasticizer or flame retardant. The other reactants, for example, water,ammonia and oxygen with which the contaminants react are present in theambient air. It is noted, that oxygen is added to the gas present in theapparatus on purpose to oxidize hydrocarbons away. The dissociation ofthe chemicals depends on factors including the volume and the partialpressure of the reactants, as well as the wavelength of the radiation.It has been found that another factor is the presence of a surface. Inparticular, it has been found that at a surface, the probability ofdissociation and hence, subsequent salt crystal growth, is higher thanaway from the surface. The embodiments shown and described withreference to FIGS. 5 to 8, in particular, exploit this last factor.

It has been found that the degradation mechanism may take one of atleast two possible paths. Two of the major mechanisms are discussed inmore detail. According to a first mechanism, the molecule contaminant ispresent in a gas (or a liquid). When subject to appropriate radiation,the contaminant dissociates to form radicals. The radicals subsequentlycollect at a surface to form nucleation sites. Crystal growth occurs asa result of rearrangement of the radicals.

According to a second mechanism, the presence of a surface in thereaction zone is necessary. A molecule is adsorbed at the surface. Eachadsorbed molecule is adhered to the surface for a certain time. Afterthe certain time, desorbtion occurs and the molecule is released fromthe surface. The amount of time a particular molecule is retained on thesurface varies from molecule to molecule according to the size and thechemistry of the molecule.

It will be understood that if a flow of a gas (or a liquid) containingthe molecule is present in a lithographic apparatus, then the chances ofnucleation and subsequent crystal growth occurring according to thefirst mechanism will be reduced with respect to a non-flow condition,because in a flowing environment, the molecule is in the vicinity of theprojection lens for only a limited period of time as it passes in theflow past the lens. Thus, if the molecule is adsorbed on the surface,the probability of nucleation is higher because it is retained in areaction zone for a longer period of time.

Whether a molecule is adsorbed on a surface will depend primarily on thepartial pressure of the molecule in a gaseous state (and theconcentration of the molecule in a liquid state). The adsorbed moleculeadheres to the surface and is subject to the following surface effect:in the reaction zone, the adsorbed molecule may be hit directly byradiation or indirectly. Indirect radiation comprises, for example,secondary electrons generated by the beam of radiation generated by thesource of the lithographic apparatus. Whereas in a lithographicapparatus, the beam of radiation may have an energy typically in theregion of about 3-100 electronvolts (eV), secondary electrons may havean energy typically in the region of about 5-10 electronvolts (eV). Inorder to cause dissociation of the adsorbed molecule, energy in theregion of approximately 5 electronvolts is required. Thus, the moleculeis dissociated either by the beam of radiation or the secondaryelectrons. The radicals, which are the product of the dissociation, formnucleation sites, further radicals are rearranged to form crystals atthe surface around the nucleation sites. Thus, according to the presentinvention, by providing a surface on which nucleation sites areprovided, the crystal growth probability is increased.

When the second mechanism described above occurs, there are two regimesthat may occur. According to the first regime, the intensity of light ishigh with respect to the partial pressure (or concentration) of themolecule. Thus, crystal growth will be determined by the partialpressure (or concentration in a liquid). This regime is referred to asthe “molecular flux limit regime”, and occurs typically at low partialpressures (or concentrations). According to a second regime, the lightintensity is low with respect to the number of molecules in the reactionzone (i.e. there is a relatively high partial pressure (orconcentration)). Thus, growth is determined by the intensity of theradiation. This regime is referred to as the “radiation flux limitedregime”, and is not limited by the partial pressure (or concentration).

It has been found that the regime which is typically encountered inlithographic apparatus is the molecular flux limited regime becausethere is a low partial pressure (or concentration). Crystal growthtypically takes place over a long period of time. In particular,however, the regime to which the present invention is applied is nothighly dependent on the intensity of the radiation. Thus, in order forthe present invention to work, although the radiation is required tohave a certain energy, it does not require a particularly highintensity. In particular, the required intensity is lower than that of atypical beam of radiation produced by the source in a lithographicapparatus.

FIG. 2 depicts details of a lithographic apparatus including aprojection system protector according to an embodiment of the presentinvention. In particular, FIG. 2 shows a projection system PL comprisingat least one lens element having an entry surface 60 and an exit surface80. In use, the (patterned) beam enters the projection system via apredetermined area of the entry surface 60, the area of the incidentbeam is the entry area 70. The beam exits the projection system via apredetermined area of the exit surface 80, the area of the exiting beamis the exit area 90. In the example shown in FIG. 2, the entry and exitarea have the shape of a cross. However, the invention is not limited inthis respect. Typically, the entry and exit areas correspond to thecross section of the patterned beam in the entry and exit planes of theprojection system, respectively, and will vary depending on theparticular application. In the example shown in FIG. 2, the entry andexit area have the same shape, however, the invention is not limited inthis respect.

Also provided is a nucleated surface 40 on which a plurality ofcontamination nucleation sites are provided, the surface being disposedaway from at least one of the beam entry area or said beam exit area.For example, a nucleated surface may be provided either at the entry tothe projection system or the exit, or both. The provision of a nucleatedsurface away from the beam path initiates crystal growth outside of thecritical path of the radiation and off the part of the lens throughwhich the patterned beam passes. Instead, crystal growth is initiated onthe prepared surfaces around the perimeter of the path of the beam. Thenucleated surface is prepared by providing nucleation sites on thesurface. Preferably, the nucleation sites are ideal, that is, thenucleation sites comprise crystal seeds of the compound which is to begrown, and are prepared and provided on the surface prior toinstallation of the surface. Preferably, the surface is rough. Thesurface roughness may have a root mean square value of about 3-5nanometers or more. For example, the nucleated surface 40 may be etched,preferably, lightly etched, quartz with ammonium sulphate salt seedsdisposed thereon. Preferably, the nucleation site is made of the samematerial as the lens element disposed either directly upstream ordownstream of the nucleated surface 40, however, this is not essential.Typical materials for the nucleation surface include any lens materialincluding, but not limited to, silicon dioxide, magnesium fluoride, andcalcium fluoride.

As can be seen in FIG. 2, the nucleated surface 40 forms part of aprotective cap 200 which is disposed in use, as shown by arrow 101, onthe end of the projection system. In FIG. 2, the protective cap 200 isshown for provision on the exit end of the projection system. However,as mentioned, it is envisaged that the protective cap may be disposed oneither the entry, exit, or both ends of the projection system.Preferably, the nucleated surfaces forms part of a protective cap whichfits over at least the exit or entry area of the projection system.Preferably, the nucleated surface 40 is disposed adjacent to either thebeam entry area or the beam exit area in the plane of these areas, orboth. The dimensions of the nucleated surface are not critical. It isadded that according this particular embodiment, it is not necessary forthe nucleated surface to be exposed to the radiation in the apparatus.Dissociation will occur in the volume of the apparatus. However, thedissociated molecules will with very high probability form salts at thenucleation sites provided on the nucleated surface 40 because thenucleation sites are seeds on which further crystal growth can occur.

FIG. 3 depicts details of a lithographic apparatus including aprojection system protector according to a further embodiment of thepresent invention. In FIG. 3, the nucleated surface 40 is formed usingat least one tube, preferably, a quartz tube. Preferably, the nucleatedsurface includes at least one tube 110 disposed on a surface 120, whichmay or may not be the external surface of the entry or exit lenselement, wherein the at least one tube 110 is arranged on the surface120, so that in use, the tube 110 is offset from the beam entry or exitarea 70, 90 (not shown in FIG. 3) in a direction of propagation of thebeam and is adjacent to either the beam entry area or the beam exitarea. The tube or tubes 110 may be attached to the lens element usingglue or some other conventional attaching means, such as attaching tothe lens body with the use of clamps or by disposing a circular piecearound the lens where the circular piece is smaller than the place oflargest diameter of the lens, so that it hangs on the lens. As can beseen from FIG. 3, it is not necessary that the tube or tubes 110surround the entire perimeter of the cross section of the beam in thedirection of its propagation through the apparatus. Preferably, however,the nucleated surface 40 in both the embodiments shown in FIGS. 2 and 3,extends around, preferably at a nominal distance from, the area of thelens element through which the beam enters and exits the projectionsystem. In use, the distance at which the nucleated surface is disposedfrom the beam will depend on the exposures carried out by a particularlithographic apparatus. In particular, if a variety of beam crosssections are envisaged, it may be beneficial to dispose the nucleatedsurface 40 a sufficient distance to allow all envisaged beam profiles tobe carried out without having to change the protective cap orarrangement of tubes on the lens elements. However, generally, for aparticular beam profile, it is preferable that the nucleated surface 40,in use, is as close to the area of incidence of the beam withoutoverlapping the exposure area in the plane of the entry and exit of thebeam. Although, in FIG. 3, the nucleated surface is shown as anarrangement of tubes, the invention is not limited in this respect.Indeed, it is envisaged that the surface may include cubic, or any othergeometric structures planted with nucleation sites.

FIG. 4 depicts details of a lithographic apparatus including aprojection system protector and attachments according to a furtherembodiment of the present invention. The nucleated surface 40 isprovided with attachment elements 140 for attaching nucleated surface 40onto said projection system PL. In the embodiment shown in FIG. 4, thenucleated surface 40 is not disposed directly on the entry or exit lenselement, but is rather disposed on a purge hood 130, which is disposedbetween the projection system PL and the nucleated surface 40.Attachment elements 140 are provided to attach the nucleated surface tothe purge hood. As with FIG. 2 or 3, the nucleated surface may compriseetched quartz on which nucleation sites are disposed, or an arrangementof tubes. However, the invention is not limited in this respect, and asmentioned above, the nucleated surface may be attached directly to alens element. The purge hood 130 is an optional component of alithographic apparatus and includes components for introducing purifiedair into the apparatus.

FIG. 5 depicts details of a lithographic apparatus including a fluidcleaning system in which a nucleated surface is incorporated accordingto a further embodiment of the present invention. For those componentsshown in FIG. 5 having the same reference numeral or letter ascorresponding components shown in FIG. 1, reference is made to thedescription of FIG. 1 above, as these components are not describedfurther hereinbelow. FIG. 5 shows a projection beam of radiation 1, apatterning device MA, serving to pattern the projection beam accordingto a desired pattern, a patterned beam 3. The patterned beam having aparticular cross sectional area is incident on a projection system PL ata projection system entry area (not shown in FIG. 5). The patterned beamexits the projection system via a beam entry area (not shown in FIG. 5).The exiting patterned beam 5 is subsequently incident on a substrate W,which is mounted on substrate table WT. FIG. 5 further shows a gascleaning system 10, 14, 15, 16, 20 according to an embodiment of theinvention which is for use with an apparatus such as the lithographicapparatus shown in FIGS. 1 and 5. Although, in the example shown, thefluid cleaning system is shown in use with a lithographic apparatus, theinvention is not limited in this respect, and it is envisaged that thefluid cleaning system according to this embodiment of the presentinvention has application in other apparatus where clean fluid isnecessary, such as wafer inspection tools, gas purification systems forbottled gases such as nitrogen or argon.

In the embodiments shown in FIGS. 5 to 8, the lithographic apparatusoperates in a gaseous environment. The fluid cleaning system cleans thegas surrounding the projection system PL. As discussed below, theinvention also has application in lithographic and other apparatusoperating in a liquid environment. Also shown in FIG. 5 is aconventional gas cleaning system 8, also referred in the art as an gaspurging system. Such a conventional gas cleaning system 8 is optional.It is envisaged that the gas cleaning system 10, 14, 15, 16, 20 shown inFIG. 5 may be used both together with a conventional system, oralternatively without a conventional gas cleaning system 8. Further, inthe embodiment shown in FIG. 5, the gas cleaning system incorporatingthe present invention is disposed upstream in the direction of the gasflow of the conventional gas cleaning system 8. However, the inventionis not limited in this respect, and it is envisaged that the gascleaning system 10, 14, 15, 16, 20 may also be disposed downstream inthe direction of the gas flow of the conventional gas cleaning system 8.The gas cleaning system has particular application to cleaning air.However, it is envisaged to be used for cleaning other gases such asnitrogen, argon, helium, neon and hydrogen, in which contaminants of thetype described above are present.

The gas cleaning system 10, 14, 15, 16, 20 according to the embodimentsshown in FIGS. 5-8 cleans (or purifies) gas, typically nitrogen, using amethod based on the principles of which cause the formation of saltcrystal contaminants on the surfaces of the lens elements. It has beenfound that the contamination of the lens element, which is a problemwith conventional systems, may be controlled in the following way.Firstly, those areas of the apparatus, typically a lithographicprojection apparatus, requiring ultra clean air are identified. Asmentioned, such areas are those above the first element in theprojection system and below the last lens element in the projectionsystem. Optionally, as shown in FIG. 8, optically transparent pellicules22, 23 may be provided to separate those parts of the apparatusrequiring ultra clean air from contamination sources. In a lithographicprojection apparatus, such areas also include, for example, the mask MAarea and the wafer W area. Secondly, a gas cleaning system 10, 14, 15,16, 20 is incorporated, either in addition, or separate fromconventional air purging systems 8, to provide ultra clean air to thoseidentified regions. By identifying and isolating those areas of anapparatus requiring ultra clean gas, it is not necessary that the gascleaning system be designed as a high throughput system. This furtherreduces the complexity and cost of the gas cleaning system required.

The gas cleaning system 10, 14, 15, 16, 20 is described in furtherdetail with reference to FIGS. 6 and 7. Typically, however, the gascleaning system includes a gas inlet 10 for receiving gas to be cleanedand a gas outlet and supply system 14 for supplying cleaned gas to theidentified regions of the apparatus requiring clean gas, a cleaning zone20 for cleaning the received gas, the cleaning zone being disposedbetween the inlet 10 and the outlet 14. The gas cleaning system furthercomprises a radiation system 15 for providing radiation 16 to thecleaning zone 20. The radiation is such that it causes dissociation of acontaminant present in the cleaning zone 20. The gas cleaning systemfurther comprises a nucleated surface provided with a plurality ofnucleation sites with which the dissociated contaminants associate,wherein the nucleated surface is disposed in the cleaning zone 20.

Typically, the radiation system has sufficient intensity, which may beless than that found in the lithographic apparatus.

The illumination of the gas to be cleaned takes place in the cleaningzone, which is preferably a specially designed chamber 20 where the gasto be cleaned is fed through. The chamber is designed so that the gasflows in the high radiation intensity area for a sufficiently largetime, or along a sufficiently long path length to achieve dissociationof a contaminant in the cleaning zone and association of the dissociatedcontaminant with the nucleated surface. Preferably, the chamber containsa large active surface area, preferably of the same material as thelenses of the lithography apparatus or other apparatus for which the gascleaner is intended, such as quartz silicon dioxide for G-line andI-line and KrF2 laser generation, magnesium fluoride for ArF2 lasergeneration and CaF2 for the F2-laser generation. Preferably, the activesurface area of the nucleated surface, or alternatively, the wholechamber 20 is provided as a removable unit, which may be easily removedfrom the gas cleaning system for maintenance and replacement purposes.Preferably, the active surface area is rough. Preferably, the surfacearea of the active area is large in comparison to the surface area ofthe lens element.

The nucleated surface may be prepared by wiping the surface with a pieceof cleaning cloth after a period of initial exposure of the chamber toradiation whilst gas passes through the chamber. This preparationinduces slow contaminating growth of salt crystals on the surface.Alternatively, the dummy surface is pretreated under similar conditions,not during operation, to achieve nucleation of the active surface. Asurface prepared in such a manner once installed in the gas cleaningsystem for an apparatus demonstrates enhanced contaminating growth onthe nucleated surfaces due to the highly increased number of growthnuclei present on the surface. It has been found that by providing highnucleation site density of contaminating salts in the gas cleaningsystem, improved gas cleaning is achieved, as more contaminants areremoved from the gas prior to its introduction into the identifiedareas, such as the area surrounding the lens elements.

In a particular embodiment, as shown in more detail in FIG. 6, the gasflow in the illumination chamber 20 is led through a system of channelsformed with walls transparent to the radiation. In the embodiment, asshown in more detail in FIG. 7, the chamber is provided with a foamed orglass wool manufactured of the appropriate material, for example, fromthe lens materials mentioned above. The provision of channels or foamedor glass wool provides a large contact surface. The dwell time of theair in the illumination chamber 20, resulting from the channel design ofFIG. 5 and the foam or glass wool filling of FIG. 6, combined with theturbulent flow dynamics of the gas, is arranged to provide sufficientcontact with the nucleated surface. Preferably, the surface reactionprobability, i.e., the probability of a contaminant dissociating andassociating on the nucleated surface as a salt crystal in theillumination chamber, is rendered as high as possible in theillumination chamber. This is achieved as described above, by optimizingthe surface area of the surface, the occurrence of nucleation sites onthe surface, and by the intensity of the radiation incident in thechamber.

The gas cleaned by the gas cleaning system according the embodiments ofthe present invention is preferably used only near the lens surfaces ofinterest present in the identified regions in such a way that no othergas is able to reach these lens surfaces. These regions can be shieldedfrom the rest of the apparatus volume by means of optically transparentpellicles 22, 23, shown in FIG. 8, which provide the possibility ofrecycling the ultra clean air and, hence, reducing the contaminationrate and probability even further in the identified regions. In aparticular embodiment of the present invention, the gas cleaning system20, 21 shown in FIG. 8 is dispensed with, and the pellicles 22, 23 areprovided with, nucleation sites which act as the surface with which acontaminant present in or around the projection system PL associates.The pellicles 22, 23 being disposed away from at least one of thepatterned beam entry area or the patterned beam exit area.

FIG. 6 depicts details of a lithographic apparatus including a fluidcleaning system according to an embodiment of the present invention.Purge gas comprising precleaned gas or recycled ultra clean air 10 isprovided via purge gas inlet 11. The cleaning zone comprises a chamber12, and is preferably provided with obstructing walls 24. The chamber isilluminated with radiation 16 from source 15. Preferably, obstructingwalls 24 are transparent to the radiation 16. The obstructing walls 24are provided with nucleation sites, and thus form the nucleated surface.Preferably, the radiation is incident on the nucleated surfaces. Inparticular, the walls are disposed so as to define a gas path throughwhich said gas passes from said gas inlet 11 to gas outlet 13. Inparticular, the walls 24 are arranged so that the gas path has a lengthlonger than a dimension of the chamber in a direction of propagation ofthe radiation. It can be seen from FIG. 5 that the radiation propagatesin the direction indicated by arrows 16. As the radiation propagatesthrough the chamber 20, the radiation continues to propagatesubstantially in the same direction. However, it will be understood thatthe radiation may experience some changes in direction of propagationdue to reflection and some absorption in the chamber. In particular, inorder to exploit the energy of the radiation 16, a mirror 17 forreflecting the radiation exiting the chamber 20 is optionally provided.In the embodiments shown in the Figures, the mirror is depicted as beinga flat mirror. However, the mirror may have any shape, including aspherical geometry. The mirror 17 reflects radiation exiting the chamber20 back into the chamber in order that it may produce furtherdissociation of contaminants present in the gas in the chamber.

Further, in the embodiment shown in FIG. 6, it is seen that the wallsare interleaved.

A contaminant in the gas passing through the cleaning zone is removed asdescribed above. The gas 14 cleaned in the chamber is outlet to anapparatus, for example a lithographic projection apparatus, via an ultraclean purge gas outlet towards the apparatus.

FIG. 7 depicts a fluid cleaning system according to a further embodimentof the present invention. Purge gas comprising pre-cleaned gas orrecycled ultra clean gas 10 is introduced into a pure gas inlet 18,which is structured to provide efficient gas flow distribution control.Preferably, the cleaning zone comprises a chamber 20, which includes anucleated surface. Preferably, the nucleated surface comprises a surfaceof foamed or glass wool disposed in the chamber. The cleaned gas 14 isfed via a purge gas outlet 19, which is structured to provide efficientgas flow distribution control, to the desired location. As in theembodiment shown in FIG. 6, a mirror 17 is provided to reflect radiationexiting the chamber back into the chamber in order to exploit the energyof the radiation to achieve optimum dissociation of contaminants in thecleaning zone.

FIG. 8 depicts details of a lithographic apparatus including a fluidcleaning system according to a further embodiment of the presentinvention. In addition to those components already described withreference to FIGS. 1 and 5, FIG. 8 shows, as also already mentioned,optically transparent pellicules 22, 23 which function to restrict thecirculation of the ultra clean gas cleaned by the gas cleaning system ofthe present invention, within the apparatus, to the desired regions. Inaddition, in the absence of the gas cleaning system 20, 21, thepellicles 22, 23 are provided with nucleation sites, so that thepellicles 22, 23 also function as the surface at which a contaminantassociates. An ultra clean gas system 20, as previously described withreference to FIGS. 5-7, is preferably provided, which outputs gas to theregion restricted by the pellicules 22, 23 in the direction of arrows25. Preferably, the gas is blown with sufficient force to traverse therestricted region, in which for example, the projecting system islocated. Provided on the opposite side of the restricted region is anultra clean gas collection unit 21. The structure of the collection unit21 is preferably the same as the delivery unit 20. However, it may havethe structure of any of the embodiments of the gas cleaning system ofthe present invention. The collection unit 21 collects the gas emittedby the delivery unit and recycles it. The recycled gas may be outputdirectly into the restricted region, or it may be fed back to thedelivery unit 20 where it is cleaned once again before beingreintroduced to the restricted region. It has been found that recyclingthe gas further reduces the contaminants present in the gas and thusfurther decreases the probability of a surface reaction occurring withinthe restricted region.

In the embodiments shown in FIGS. 5-8 it is seen that that the cleaningzone is disposed away from the patterned beam. Further, it is understoodthat the chamber is constructed so that the time it takes for the gas topass from the inlet to the outlet is sufficient to achieve dissociationof a contaminant in the cleaning zone. Preferably, the nucleated surfaceis constructed so that the time it takes for the gas to pass from theinlet to the outlet is sufficient to achieve association of thedissociated contaminant at the nucleated surface. In particular, withreference to the application in a lithographic apparatus, the surfacearea of the nucleated surface is greater than the cross sectional areaof the patterned beam.

As mentioned previously, the nucleation sites are salt crystal growthseeds and the association includes the formation of salt crystals at orin the vicinity of the nucleation sites. In particular, the contaminantsare retained on the nucleated surface as salt crystals.

Further, the first radiation system providing the patterned beam and thesecond radiation system providing the dissociating radiation in theembodiments shown in FIGS. 5 and 8 may be the same. However, in analternative embodiment, these radiation systems are independent fromeach other. The wavelength of the radiation provided by the secondradiation system is not important, provided it causes dissociation of aparticular contaminant. In one embodiment, the first and secondradiation system provide radiation having substantially the samewavelength.

In FIG. 8, the cleaning unit 20 is shown as delivering clean air to onestepper unit. However, the invention is not limited in this respect, andit is envisaged that a single cleaning unit 20 may be used to supplyclean air to a number of steppers, for example, steppers disposed inparallel to one another.

In the embodiments described above, reference is made to lithographicapparatus which operate in a gaseous environment. However, the inventionis not limited to such apparatus. The present invention can be used toremove contaminants from fluids. Thus, a contaminant present in either agas or a liquid will associate with the nucleated surface. Inparticular, the present invention also has application to immersionlithographic apparatus, i.e., lithographic apparatus which operate in apartially liquid environment. In immersion lithography, the spacebetween the last lens of the projection system and the wafer is filledwith a liquid. The liquid is typically water or oil. An example, of atypical oil is Fomblin, which is also used as pump oil. The presentinvention has application to clean the used water or oil before use orcleaning the fluid present between the last lens and the wafer. It hasbeen found that the contamination of lenses is even greater in immersionlithography than in standard lithography.

In addition, the use of a “dummy” nucleated surface, i.e., a surface onwhich a plurality of nucleation sites are provided, may be used as adetector. For such an application, the nucleated surface is disposed inthe fluid path, for example, at a fluid inlet where the fluid enters theapparatus. The nucleated surface is disposed within the path of aradiation beam of sufficient energy to cause crystal growth in thepresence of a contaminant. The fluid to be tested is then passed overthe surface. If the fluid contains a contaminant or contaminants, overtime crystal growth will occur. The amount of crystal growth is relatedto the concentration of contaminants. Since the crystal growth affectsthe optical properties of the nucleated surface, the amount of growth,and hence the concentration of contaminants can be determined optically.Thus, a nucleated surface disposed in a radiation beam can be used todetect the concentration of contaminants in a fluid flowing along afluid path, wherein the detecting surface is disposed. If it is foundthat the concentration of contaminants is high, as measured, theoperation of the apparatus could be interrupted in order to replace thecontaminated fluid and to prevent further contamination of elementswithin the apparatus, such as the lens elements in a lithographicapparatus. In this way, the nucleated surface can be used as a qualitymeasure for the fluid in an apparatus. The optical detection may be donein reflection or transmission. Also, dark field methods can be appliedwhere the direct beam is blocked and only the stray light is detectedresulting in a much higher sensitivity. In particular, it is envisagedto provide a contamination detector for detecting contaminants in afluid, the detector comprising: a fluid path along which the fluid fortesting flows; a detection zone disposed in the fluid path; and aradiation source arranged, in use, to be incident on the detection zone;wherein the radiation source causes dissociation of a contaminantpresent in the fluid in the detection zone; and wherein the detectorfurther comprises: a nucleated surface disposed in said detection zone,on which a plurality of nucleation sites are provided with one of whichsaid dissociated contaminant associate and an optical measuring devicefor determining an optical characteristic of said nucleated surface fromwhich the concentration of a contaminant in said fluid is determinable.Preferably, the characteristic which is measured is transmissivity orreflectivity of the nucleated surface.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic projection apparatus comprising: a radiation systemthat provides a beam of radiation; a support structure that supports apatterning structure, the patterning structure configured to pattern thebeam of radiation according to a desired pattern; a substrate supportthat supports a substrate; a projection system that projects thepatterned beam onto a target portion of the substrate, said projectionsystem including an optical element that has a beam entry area and anoptical element that has a beam exit area through each of which saidpatterned beam passes; and a nucleated surface associated with saidprojection system on which a plurality of nucleation sites are provided,said surface being disposed away from at least one of said beam entryarea and said beam exit area.
 2. A lithographic projection apparatusaccording to claim 1, wherein said nucleated surface is made of the samematerial as at least one of said optical elements.
 3. A lithographicprojection apparatus according to claim 1, wherein said nucleatedsurface comprises quartz.
 4. A lithographic projection apparatusaccording to claim 1, wherein said nucleated surface comprises a roughtexture.
 5. A lithographic projection apparatus according to claim 1,wherein said nucleated surface includes at least one tube disposed on asurface, wherein said at least one tube is arranged on said surface sothat said tube is offset from said beam entry or exit area in adirection of propagation of said patterned beam and is adjacent toeither said beam entry area or said beam exit area.
 6. A lithographicprojection apparatus according to claim 1, wherein said nucleatedsurface forms part of a protective cap which fits over at least saidexit or entry area of said projection system.
 7. A lithographicprojection apparatus according to claim 6, wherein said cap is providedwith attachment elements for attaching said protective cap onto saidprojection system.
 8. A lithographic projection apparatus according toclaim 6, wherein a purge hood is disposed between said protective capand said at least one of said entry or exit areas of said projectionsystem.
 9. A lithographic projection apparatus according to claim 1,wherein said nucleated surface is disposed in a gas cleaning system. 10.A device manufacturing method comprising: projecting a beam ofradiation; patterning the beam of radiation; projecting the patternedbeam of radiation using an optical element having a beam entry area andan optical element having a beam exit area through each of which saidpatterned beam passes, onto a target portion of the layer ofradiation-sensitive material; and capturing contaminants with aplurality of nucleation sites spaced from at least one of said beamentry area and said beam exit area.